A landslide victory for Barack Obama in North Carolina and a nail-biter win for Hillary Rodham Clinton in Indiana - where she had been expected to prevail easily - turned Tuesday night's Democratic primaries into a momentum-killer for the New York senator, who seems to be falling even further behind in her quest for the party's presidential nomination.

And even as voters were still casting ballots in the nation's final big-state primaries, Clinton launched a new tactic, suggesting that the "finish line" for the nomination has shifted.

Obama earned a decisive, double-digit victory in North Carolina, where 115 delegates were at stake. Winning in the Tar Heel State appeared to be a strong sign that he was back on track after his campaign appeared nearly derailed by the controversy regarding his former pastor, the Rev. Jeremiah Wright.

Clinton, pounding populist themes like a gas-tax holiday and underscoring her connections with working-class voters, delivered another back-to-the-wall victory in Indiana on Tuesday, squeaking out a 51-49 percent win, a 12,000-vote margin over Obama in a state with 72 delegates up for grabs.

The Obama campaign quickly downplayed Clinton's showing in Indiana, seizing on exit polls that suggested she might have gotten a boost from the "Limbaugh effect" - as an estimated 11 percent of Republicans said they crossed over to vote Democratic, the vast majority for the New York senator. Conservative talk-show host Rush Limbaugh earlier this year launched what he calls "Operation Chaos" to try to skew the Democratic primary vote totals, and he told The Chronicle Tuesday night he was "extremely proud" of the results in the most recent primaries.

With six contests left to go in the calendar, Obama's definitive win in North Carolina, which boosted both his popular vote and delegate count, puts Clinton further behind in the endgame for the nomination. And it is likely to weaken her case with potential campaign donors and undecided superdelegates alike.

Both sides see victory

Still, both candidates tried to declare some form of victory from Tuesday's outcome.

"Not too long ago, my opponent made a prediction ... Indiana would be the tie-breaker," Clinton said, speaking to a sea of supporters in Indiana.

"Tonight, we've come from behind, we've broken the tie, and thanks to you, it's full speed on to the White House," she yelled to cheers before the final official results were even announced. "Tonight is your victory."

Obama - addressing ecstatic supporters in North Carolina - aimed his victory message directly at superdelegates as he exulted in his solid win in "a big state, a swing state, a state where I will compete to win if I am the Democratic nominee."

He also congratulated Clinton for a win in Indiana before the results were announced - but he added to cheers, "Tonight we stand less than 200 delegates away from securing the nomination for president of the United States."

With that, he issued a direct challenge to a new effort by Clinton to change the game regarding the number of delegates needed for the nomination - a number the Democratic National Committee has repeatedly said is 2,025.

Before Tuesday's two primaries, Obama had 1,745.5 delegates, compared to 1,608 for Clinton, with 2,025 having been defined by party officials as needed for the nomination, the Associated Press reported.

But asked at a campaign event Tuesday to define the "finish line," the New York senator said, "I think it's 2,209." That number would mean seating delegations from Florida and Michigan, states from which delegates have been barred from the nominating convention because both defied party rules and held early primaries.

Rules panel to meet

Democratic National Committee Chairman Howard Dean, speaking on MSNBC Tuesday, said the party's rules committee would take up the matter on May 31, but he warned that "if you're going to change the rules ... you really have to do it with the cooperation of both campaigns."

Without a sweep of primary wins Tuesday, and no real shift in momentum, Clinton's case for the nomination could be tougher to make - even as she suggests the battle will continue to rage, possibly all the way until the Democratic National Convention in August.

"The central dynamic of the race is ... unchanged: that Sen. Obama is winning and Sen. Clinton is losing," said Simon Rosenberg, who heads the Washington, D.C.-based think tank NDN, formerly the New Democrat Network. "The superdelegates have been speaking with their feet, making it clear where they're headed; he's won most of the superdelegates since March 4, and this race is really very much in his grasp."

But, Rosenberg warned, "he has to go from winning the race to actually having won it, and that's the drama. He hasn't done it - in part Sen. Clinton has run a ferocious and remarkable campaign."

Exit polling in both the Tar Heel and Hoosier states appeared to underscore patterns of strengths seen with both candidates in earlier contests.

Clinton got the upper hand with overwhelming support of white voters, seniors, Catholics and rural and small-town voters. She also won two-thirds of voters who decided in the last three days.

Another possible factor: the "Limbaugh" voters or the "mischief voters," Republicans who have followed the conservative talk-show host's call. The nation's biggest radio star has been pushing a drive he calls "Operation Chaos," urging his listeners to prolong the Democratic battle by re-registering and voting for Clinton and suggesting they lie to exit pollsters to confound the media.

Exit polls showed 11 percent of the Indiana voters Tuesday identified themselves as Republicans, and they favored Clinton 52-46 percent; but 58 percent of the Republicans who voted also said Obama is more likely to beat presumed GOP nominee Sen. John McCain in the fall, the polls showed.

Limbaugh, in an e-mail late Tuesday to The Chronicle, rejoiced in speculation from within the Obama campaign that he was responsible for as much as 7 percent of the Clinton vote in Indiana.

"I'm extremely proud of the Operation Chaos volunteers," he said. "I never doubted that they would triumph in Indiana. I think it is a hoot the Obama campaign is crediting (us) for giving Hillary a 7-point bump. It sounds like they are jealous I out-organized them."

Obama again had the advantage with the near-complete support of African Americans, as well as the majority of independent, highly educated, urban and young voters.

Exit polling showed less than half of the voters in both states viewed the Wright controversy as an "important factor" in their vote.

Obama's case

Obama supporters said his rebound from two difficult weeks makes a case to the superdelegates as much as Clinton's efforts to change the rules.

"She's been moving the goalposts all the time," said Debbie Mesloh, the San Francisco-based spokeswoman for the Obama campaign who is already working in West Virginia, the next primary on the calendar. "The prolonged nomination process is hurting (Democrats) ... so when does it end, and when does the Democratic Party really start preparing itself to run against McCain?"

But Roger Salazar, a leading Sacramento-based Democratic strategist and Clinton supporter, said the New York senator is merely showing her fighting spirit and toughness - qualities that will serve the party in November.

"At the end of the day, having a hard-fought primary and showing the political world that you're tough enough to stick it out ends up benefiting the Democratic primary as a whole," he said. "No matter what happens, we'll all come together and take out John McCain in November."

Still, some Democrats suggest that Tuesday night's outcome keeps the pressure on the party - and the candidates.

"We're back to arguing about math and likely to see renewed interest in clearing things up by having some sort of re-vote in Michigan and Florida," said San Francisco-based Democratic strategist Dan Newman.